1. Field of the Invention
The present invention relates to a nuclear detection process with base potential correction and to a corresponding apparatus. The apparatus can e.g. be a gamma-camera or spectrometer, or any other apparatus serving to measure a characteristic of a nuclear radiation. In the first case, the invention is applied to the medical field, where gamma-cameras are used for producing images of organs with a view to making a diagnosis. The gamma-camera is preferably of the scintillation, ANGER type, whereof U.S. Pat. No. 3,011,057 describes the fundamental operation and the realization means.
2. Description of the Background
Gamma-cameras are used in nuclear medicine for displaying the distribution within an organ of molecules labelled by a radioactive isotope previously injected into the patient. A gamma-camera generally comprises a collimator for limiting the angle of incidence of the gamma photons emitted by the organ to be studied, a scintillation crystal for transforming the gamma photons into light photons or scintillations and a network of photomultiplier tubes, which transform the scintillation into electrical pulses called "electrical contributions" of the tubes. A gamma-camera also comprises electronic circuits for producing, on the basis of the electrical contributions of the tubes, X and Y coordinate signals indicating the location where the scintillation has taken place, as well as a validation signal V when the amplitude of the scintillation belongs to a predetermined energy band.
This detection chain can be followed by a display system generally incorporating an oscilloscope controlled by X and Y coordinate signals and by the validation signal V for displaying by a spot of light or scan spot on the screen the impact point of the gamma photon on the crystal. The display system can optionally incorporate a photographic device for forming an image of the organ observed, by the integration of a large number of light points produced on the screen. Moreover, it can comprise a digital image processing device, which can be used for reconstructing images of sections of organs, in order to produce tomograms thereof. In the latter case, image reconstruction algorithms identical to those used in tomo-scanning are utilized.
Among other things a gamma-camera must have a good spatial resolution (i.e. a capacity to distinguish small radioactive sources which are close together), a good counting rate response (i.e. a capacity to process a large number of events per time unit) and an image quality independent of the energy of the considered isotope. The spatial resolution is dependent on the accuracy of the calculation of the X and Y coordinates. The quality of the production of these coordinates is essentially dependent on the physical laws governing the operation of the different parts of the gamma-camera. Thus, the interaction of a gamma photon with the crystal gives rise to a light scintillation, whose intensity decreases exponentially with time. The time constant of this decrease is a characteristic of the scintillation crystal used. For a thalium-activated sodium iodide crystal (NaI, Tl), it is approximately 300 nanoseconds. For a given energy of the incident gamma photon, the number of light photons of the scintillation obeys the statistical POISSON'S law. This scintillation is seen by several multiplier tubes simultaneously. The light photons forming this scintillation remove photoelectrons from the photocathodes of the photomultiplier tubes. The number of photoelectrons removed also obeys the POISSON'S statistical law for a given scintillation. This means that the electrical contribution of a photomultiplier tube receiving a scintillation has an amplitude, whose value follows a POISSON'S statistical distribution. The mean value of this amplitude is a function of the energy of the incident light photons.
As a scintillation is seen by several photomultiplier tubes simultaneously, the determination of the location of this scintillation on the crystal, which itself represents the emission point of the exciting gamma photon, is obtained by calculating the location of the barycentre of the electrical contribution supplied by the photomultiplier tubes. In accordance with the ANGER method referred to hereinbefore, this calculation takes place by injecting the electrical contributions through an array of resistance matrixes. The values of the latter are a function, for so-called pinpointing matrixes, of the positions of the photomultiplier tubes to which they are connected. The analog electrical signals obtained x.sup.+, x.sup.-, y.sup.+, y.sup.-, called "weighted pulses" translate the position of the scintillation with respect to the X and Y axes.
For a given scintillation, the most difficult problem to solve consists of determining as accurately as possible the integral of the barycentre of the electrical contributions on a period of approximately three times the decay time constant of the scintillations of the scintillation crystal. The integration time is dependent on the time constant of the crystal. The accuracy of the measurement is subject to error due to the statistical POISSON'S fluctuation. Thus, the standard variation of the amplitude fluctuation of the contributions according to POISSON'S statistics is inversely proportional to the square root of the number of photoelectrons removed. Thus, the longer the integration (up to three times the decay time constant of the scintillation) the larger the number of photoelectrons taken into account, the smaller the standard variation and therefore the more accurately the mean value of this contribution is evaluated.
Thus, the operation of calculating the location of the barycentre is a linear operation, so that it is more economic to carry out this integration at the output of the each of the resistance matrixes of the array on weighted pulses. The integration time is directly linked with the quality of the spatial resolution of the gamma-camera and this quality is obtained to the detriment of the counting rate, i.e. to the number of events per second which are taken into account.
This integration operation is subject to certain difficulties. The most important is the presence of permanent d.c. voltages super imposed on the weighted pulses and which, introduced into integrators, falsify the value of the signal supplied by them in proportion to the length of the integration time. The origin of these d.c. voltages is mainly the variable gain amplifiers interposed between each resistance matrix and a corresponding integrator. These variable gain amplifiers are used for two reasons. Firstly they are used for choosing the energy range to be studied and then they permit an amplitude matching of the weighted pulses to the operating dynamics of the integrators used. These disturbing d.c. voltages can also have other origins and can in particular be due to a scintillation stacking effect.
The electrical potential resulting from these d.c. voltages shifts is what is commonly called the base potential of the integrators. U.S. Pat. No. 3,984,689 granted on Oct. 5, 1976 to Roger E. ARSENAUX indicates that at high radioactivity levels, i.e. high counting rates, e.g. exceeding 100,000 events per second, capacitive couplings, which might have been considered for eliminating the said d.c. voltages, must be proscribed. Thus, the presence of such capacitive couplings leads to a displacement of the base potential essentially linked with the repeated appearance at very high speed of scintillations. These capacitances have the consequence of reconstituting a d.c. component dependent on the counting rate. However, the accuracy levels presently required with respect to the calculation of the coordinate signals makes it necessary for the erratic amplitude variations of the signals to remain below 1/1000th of their amplitude.
FR-A-2 546 632 proposes solving the problems created by the introduction of a capacitive coupling in the chain, whilst restoring the base potential prior to the appearance of weighted pulse to be taken into account. According to this document, the measurement of the base potential is only authorized after the end of a period where there have been no pulses and the base potential is restored prior to a new integration during the end of said period.
The disadvantage of this procedure is that when the camera is used at a high counting rate, i.e. when the gamma photon emission activity is high, the pulse-free periods are too short to enable the base potential measurement to take place under satisfactory conditions. This leads to the presence of a d.c. component during the integration and falsifies the result.